1. Field of the Invention
This invention generally relates to wireless communication antennas and, more particularly, to an improved inverted-F antenna including coplanar and coupled-feed features.
2. Description of the Related Art
Many conventional wireless communications devices, such as a wireless telephone, uses a whip antenna. The whip propagates excellently, when deployed (extended) from the device chassis. However, the antenna can have a fairly large form factor and, when seated in the chassis (contracted), performance is poor. A smaller, internally mounted, antenna is desirable. One such antenna is the so-called inverted-F antenna.
FIG. 1 is a depiction of a conventional planar inverted-F antenna (prior art). As noted in U.S. Pat. No. 6,317,083 (Johnson et al.), a planar inverted-F antenna (PIFA) may comprises a flat conductive sheet supported a height above a reference voltage plane, such as a groundplane. The sheet may be separated from the reference voltage plane by an air dielectric, or supported by a solid dielectric. A corner of the sheet is coupled to the ground via a grounding stub and provides an inductive load to the sheet. The sheet is designed to have an electrical length of λ/4 at the desired operating frequency. A feed is coupled to an edge of the flat sheet adjacent the grounded corner. The feed may comprise the inner conductor of a coaxial line. The outer conductor of the coaxial line terminates on and is coupled to the ground plane. The inner conductor extends through the ground plane, through the dielectric (if present) and to the radiating sheet. As such, the feed is shielded by the outer conductor as far as the groundplane but then extends, unshielded, to the radiating sheet.
The PIFA forms a resonant circuit having a capacitance and inductance per unit length. The feed point is positioned on the sheet a distance from the corner such that the impedance of the antenna at that point matches the output impedance of the feed line, which is typically 50 ohms. The main mode of resonance for the PIFA is between the short circuit and the open circuit edge. Thus, the resonant frequency supported by the PIFA is dependent on the length of the sides of the sheet and to a lesser extent the distance and the thickness of the sheet.
Planar inverted-F antennas have found particular applications in portable radio devices, e.g. radio telephones, personal organizers, and laptop computers. Their high gain and omni-directional radiation patterns are particularly suitable. Planar antennas are also suitable for applications where good frequency selectivity is required. Additionally, since the antennas are relatively small at radio frequencies, the antennas can be incorporated into the housing of a device, thereby not distracting from the overall aesthetic appearance of the device. In addition, placing the antenna inside the housing means that the antenna is less likely to be damaged.
However it is difficult to design a planar antenna that offers performance comparable to that of a whip antenna, in particular as far as the bandwidth characteristics of the device are concerned. Loss in an antenna is generally due to two sources: radiation, which is required; and energy that is conducted away from the antenna, which is undesirable. Planar antennas have an undesirably low impedance bandwidth.
It would be advantageous if the conventional inverted-F antenna could be improved to reduce its form factor and enhance its gain.
It would be advantageous if an inverted-F antenna could operate with another antenna in a non-interfering manner, while sharing a common ground plane.